The present invention generally relates to a microwave power amplifier using phase inverters, and more particularly to a microwave power amplifier such that a microwave input signal is distributed to unit amplifiers and output signals supplied therefrom are combined.
Recently, there has been considerable activity in the development of a microwave range high-output amplifier of the solid-state type. Generally, an input signal is distributed to unit amplifiers, the outputs of which are combined.
Referring to FIG. 1A, there is illustrated a conventional microwave power amplifier. An input signal in the microwave range is applied to a directional coupler 1 having a distribution loss of 3 dB and causing a 90.degree. phase difference between two output signals. One of the two output signals from the directional coupler 1 is in phase with the input signal, and the other output signal is 90.degree. out of phase. The two 90.degree. out-of-phase output signals from the directional coupler 1 are respectively applied to unit amplifiers 2 and 3. Output signals from the amplifiers 2 and 3 are combined by a directional coupler 4, which generates an amplified output signal. When the input signal is reflected by the amplifiers 2 or 3, a reflected signal component is absorbed by a terminal resistor provided in each of the directional couplers 1 and 4. Thus, the return loss is small.
FIG. 1B illustrates another conventional microwave power amplifier of the Wilkinson type. An input signal is divided into two in-phase signals by a Wilkinson type power divider 5. The two signals from the power divider 5 are individually amplified by unit amplifiers 6 and 7. The output signals from the unit amplifiers 6 and 7 are combined by a Wilkinson type power divider 8 which generates an amplified output signal.
It is assumed that output power of each of the unit amplifiers 2, 3, 6 and 7 is represented by P0 (Watt). An insertion loss generated in each of the elements 1, 4, 5 and 8 when a signal passes therethrough once is represented by L1 (dB). In this case, the output power of each of the configurations shown in FIGS. 1A and 1B is 2P0.multidot.10.sup.-(L1/10) (W). The output power of an extended configuration such that a circuit identical to the circuit shown in FIG. 1A or 1B is connected in parallel thereto, is 4PO.multidot.10.sup.-(L1/10).spsp.z (W). FIG. 2 illustrates output power characteristics of the above-mentioned configurations. FIG. 2 also illustrates the output voltage characteristic of a simplified configuration in which a single unit amplifier is used. The output power of this configuration is P0 (W).
However, the conventional microwave power amplifiers shown in FIGS. 1A and 1B have a disadvantage in that a large power loss occurs in each of the elements 1, 4, 5 and 8. The gain of each of the configurations shown in FIGS. 1A and 1B is G0-2L1 (dB) where G0 (dB) is the gain of each of the unit amplifiers 2, 3, 6 and 7. Further, the gain of the aforementioned extended configuration is G0-4L1 (dB), and the gain of the aforementioned simplified configuration is G0 (dB). FIG. 2 also illustrates gain characteristics of the individual configurations. It can be seen from the graph of FIG. 2 that the gain of each conventional configuration is decreased by the loss generated in the directional couplers 1 and 4 or the power dividers 5 and 8. Further, it can be seen from the graph of FIG. 2 that the loss of gain decreases with an increase in the number of signals to be distributed and combined.